js如何合并两个有序数组

Array.prototype.mergeSort = function () {

  const rec = (arr) => {

    if (arr.length === 1) {

      return arr;

    }

    const mid = Math.floor(arr.length / 2);

    const left = arr.slice(0, mid);

    const right = arr.slice(mid, arr.length);

    const orderLeft = rec(left);

    const orderRight = rec(right);

    const res = [];

    while (orderLeft.length || orderRight.length) {

      if (orderLeft.length && orderRight.length) {

        res.push(

          orderLeft[0] < orderRight[0] ? orderLeft.shift() : orderRight.shift()

        );

      } else if (orderLeft.length) {

        res.push(orderLeft.shift());

      } else if (orderRight.length) {

        res.push(orderRight.shift());

      }

    }

    return res;

  };

  const res = rec(this);

  res.forEach((n, i) => {

    this[i] = n;

  });

};

const arr = [5, 4, 3, 2, 1];

arr.mergeSort();